The mammalian ear functions by transforming sound waves, or airborne vibrations, into electrical impulses. The brain then recognizes these electrical impulses as sound. The ear has three major parts, the outer, middle, and inner ear. Sound waves enter the outer ear and cause the eardrum to vibrate. The vibrations of the eardrum are transmitted serially through the three ossicles in the middle ear—the malleus, incus and stapes, also called the hammer, anvil and stirrup, respectively. The stirrup transmits the vibrations to the inner ear. The inner ear comprises the cochlea and is connected to the middle ear via the oval and round windows. The inner ear is filled with fluid and vibrations transmitted to the inner ear cause fluid movement in the cochlea of the inner ear. Fluid movement in the cochlea causes movement of sensory hair cells which initiates nerve impulses. These nerve impulses are interpreted in the brain as sound.
The sensory hair cells are contained in the organ of Corti, which coils around the inside of the cochlea. Within the organ of Corti there are inner and outer sensory hair cells. The outer sensory hair cells are present in three rows, designated OHC1, OHC2 and OHC3; inner sensory hair cells are present in one row. The sensory hair cells are attached to the basilar membrane and contact the tectorial membrane. Movement of fluids within the inner ear causes a movement of the basilar membrane relative to the tectorial membrane. This relative movement causes the cilia on the sensory hair cells to bend and leads to electrical activity. Cochlear ganglion neurons below the sensory hair cells transmit this electrical activity to auditory regions of the brain via the auditory nerve.
The fluid filled inner ear, also called the membranous labyrinth, further contains the two mammalian organs of equilibrium which make up the vestibular system. The first organ of equilibrium is composed of the saccule and utricle which detect and convey information on body position relative to gravitational force. Both the saccule and utricle also contain sensory hair cells. Tiny particles of calcium carbonate lie on the sensory hair cells in the saccule and utricle and bend the cilia to stimulate the sensory hair cells to send appropriate signals to the brain, including “up”, “down”, “tilt” and “acceleration” in a particular direction. Sensory hair cells in the utricle detect linear movement in the horizontal plane while sensory hair cells in the saccule detect movement in the vertical plane.
The second organ of equilibrium is composed of three semicircular canals which detect and convey information on movement, detected as fluid acceleration, to the brain. The semicircular canals are also lined with sensory hair cells, and are arranged at near 90 degree angles with respect to one another and can detect movement in three dimensions. As the head is accelerated in one of these planes, fluid movement in the canal corresponding to the plane of movement stimulates movement of the cilia of the sensory hair cells.
The vestibular organs—the saccule, the utricle and the semicircular canals—stimulate nerve endings of vestibular ganglion neurons which then transmit information to a number of sites for different purposes. For example, information is transmitted from the vestibular system to the eyes to keep the eyes focused on a target while the body is moving. Neurons also interconnect the vestibular system and the cerebellum for producing smooth and coordinated bodily movements. Vestibular information also travels down the spinal cord to muscles in order to maintain proper posture and balance.
Significant hearing loss causing communication problems occurs in about ten percent of the population and more than one third of us will have substantial hearing loss by old age. Noise-induced hearing loss is estimated to be the cause of hearing loss in about one-third of the 28 million Americans with hearing loss (NIH Publication No. 97-4233, April 1997). In most cases, the auditory impairment results from the death of sensory hair cells in the organ of Corti. Sensory hair cells are delicate cells and thus are susceptible to damage from several sources, including, but not limited to, noise, infection, drugs, vascular insufficiency and idiopathic effects. Idiopathic effects are those effects which arise spontaneously or from an unknown or obscure cause.
Presbycusis is age-related hearing loss. Four distinct types of presbycusis have been described which are based upon audiograms and pathological analyses: 1) sensory—loss of sensory hair cells and secondary degeneration of cochlear neuronal structures, 2) neural—loss of cochlear ganglion cells and/or nerve, 3) metabolic—atrophy of the stria vascularis, and 4) mechanical—stiffening of the basilar membrane (Schuknecht, Arch. Otol., 80: 369-382, 1964). The neural and metabolic causes of presbycusis may also result in the ultimate loss of hair cells.
While no frequency data is associated with the descriptions of the types of presbycusis, sensory presbycusis is the most common (Working Group on Speech Understanding and Aging, Speech understanding and aging, J. Acoust. Soc. Am. 83: 859-895, 1988). Johnsson et al. have described both degeneration of the stria vascularis and hair cell loss in 150 patients ranging in age from newborn to 97 years of age. Both are progressive and most pronounced in elderly subjects. An age-related loss of hair cells of the vestibular apparatus—saccule and utricle—was also noted that may account for vestibular disturbances in the elderly (Johnsson et al., Ann. Otol. Rhinol. Laryngol. 81: 179-193, 1972; Johnsson et al., Ann. Otol. Rhinol. Laryngol. 81: 364-376, 1972).
We are born with a complement of about 16,000 sensory hair cells and 30,000 auditory neurons in each ear. These cells do not regenerate during postnatal life. Therefore, loss of each cell, due to, for example, noise, infection, toxic drugs (such as platinum-based cytotoxic agents and aminoglycosides) or idiopathic effects is irreversible and cumulative. If enough sensory cells are lost, the end result can be total deafness.
Noise trauma is a widespread cause of hearing loss. Sound overexposure has been demonstrated to lead to sensory hair cell apoptosis in the avian inner ear (Nakagawa et al., ORL, 59: 303-310, 1997). There is increasing evidence that the death of sensory hair cells caused by drugs such as platinum-based cytotoxic agents and aminoglycosides is partially, if not mainly, apoptotic. Noise-induced sensory hair cell loss in the cochlea apparently has a similar mechanism.
Aminoglycosides are widely used antibiotics used in patients with Gram-negative bacterial infections (Paparella et al, Otolaryngology, 1817, Saunders-Philadelphia, 1980). Aminoglycosides are known to cause damage to sensory hair cells and thereby affect hearing. Aminoglycosides include, but are not limited to, neomycin, kanamycin, amikacin, streptomycin and gentamicin. Amikacin causes apoptosis of sensory hair cells in rat cochleas (Vago et al., NeuroReport 9: 431-436, 1998). Gentamicin treatment results in degeneration of sensory hair cells in guinea pigs (Li et al., J. Comparative Neur., 355: 405-417, 1995; Lang et al., Hearing Res., 111: 177-184, 1997).
The loss of sensory hair cells in the cochlea has been attributed to aminoglycoside ototoxicity. Apoptosis of sensory hair cells of guinea pigs was observed following chronic treatment with aminoglycoside (Nakagawa et al., Eur. Arch. Otor., 254: 9-14, 1997; Nakagawa et al., Acta Otol., 255(3): 127-131, 1998). Studies have assessed the protective effect of various polypeptides on sensory hair cells in the cochlea. (See, for example, Malgrange et al., Abstr. Assoc. Res. Otol., 17: 138, 1994; Low et al., J. Cell. Physiol. 167: 443-450, 1996; and Ernfors et al., Nature Medicine, 2: 463-467, 1996). Ernfors et al. noted that, although the peptide NT-3 is a potent factor for preventing the degeneration of spiral ganglion neurons, NT-3 “insufficiently protects the hair cells” (Ernfors et al., Nature Medicine, 2: 463-467, 1996).
Platinum-based cytotoxic agents include, but are not limited to, cisplatin and carboplatin. Cisplatin is a widely used antitumor drug which causes structural changes in the inner ear and peripheral sensory neuropathy. Hearing loss due to cisplatin is usually permanent and cumulative.
Rapid onset hearing loss, also named sudden sensorineural hearing loss, may also occur without any obvious reasons. Hearing loss in these situations develops either instantaneously or after a few hours. The location of the damage is within the cochlea, and has been partially attributed to sensory hair cell damage. Such hearing loss may be due to idiopathic causes or as a result of other causes, including vascular disease, hypertension and thyroid disease and viral infection by viruses including mumps, measles, mononucleosis, adenovirus, (Thurmond et al., J. La. State Med. Soc., 150: 201-203, 1998).
Damage to sensory hair cells and cochlear neurons may also occur as a result of infection. For example, the onset of meningitis has been linked to hearing loss as a result of damage to sensory hair cells. (Blank et al., Arch. Otol. Head Neck Surg., 120: 1342-1346, 1994). Meningitis as a result of E. Coli infection also damages sensory hair cells (Marwick et al., Acta Otol. (Stockholm), 116(3): 401-407, 1996). Toxins from Streptococcus pneumoniae have also been linked to damage to sensory hair cells (Comis et al., Acta Otol. (Stockholm) 113(2): 152-159, 1993).
Accessory epithelial structures of the cochlea and innervating cochlear neurons stay intact for a considerable length of time following trauma, but undergo secondary retrograde degeneration following the loss of IHCs (Ylikoski et al., 1974; Hawkins, 1976).
Several authors have recently shown that the cochlear sensory hair cells can be protected to some extent from both ototoxic and noise damage using various compounds. This was shown in animal model systems using hair cell counts and hearing threshold measurements, e.g. by auditory brainstem responses. The most commonly used therapeutic compounds have been antioxidants or free radical scavengers.
In addition to immediate mechanical damage, oxidative stress associated with the formation of free radicals (see discussion) and excitotoxicity (Basile et al., Nature Med. 2:1338-1343, 1996) have been implicated in the pathogenesis of hearing loss. Evidence in various cell lines and in vivo neuronal and non-neuronal model systems shows that apoptotic death can be induced by both oxidative stress and excitotoxicity (reviewed by Pettmann and Henderson, Neuron 20:633-647, 1998). In the inner ear, necrotic hair cell death, characterized by cellular swelling, has been demonstrated following acoustic trauma (Kellerhals, Adv. Oto-Rhino. Laryng. 18:91-168, 1972). More recent data, obtained in the ototoxic drug-damaged inner ear, have suggested that hair cells may also die through apoptosis, based on the observations of nuclear fragmentation (Forge, Hear. Res. 19:171-182, 1985; Lee et al. J. Comp. Neur., vol. 355, 405-417, 1995; Liu et al., Neuroreport 9:2609-2614, 1998; Nakagawa et al., Eur. Arch. Otorhinolaryngol. 255:127-131, 1998; Vago et al., NeuroReport 9:431-436, 1998). However, the contribution of apoptotic hair cell death to the loss of hearing function is not known. In addition, the molecular mechanisms involved in commitment to hair cell death are unknown.
Antioxidants and free radical scavengers have been tested because both ototoxic drug and noise damage have been postulated to produce an excess of reactive oxygen species (ROS) in the inner ear. Overproduction of ROS is thought to cause sensory hair cell damage by overwhelming the cochlea's antioxidant defense system (Ravi et al., Pharmacology and Toxicology 76: 386-394, 1995).
One of the signaling cascades that has been shown to mediate apoptotic death in response to a variety of stressful stimuli is the c-Jun-N-terminal kinase (JNK) pathway, also known as the stress-activated protein kinase (SAPK) pathway (Dérijard et al., Cell 76:1025-1037, 1994; Kyriakis et al., Nature 369:156-160,1994). JNK activation by phosphorylation has been shown to be important for neuronal cell death after trophic factor withdrawal in vitro and after injury in vivo (Xia et al., Science 270:1326-1331, 1995; Dickens et al., Science 277:693-696, 1997; Yang et al., Nature 389:865-870, 1997). JNKs in turn phosphorylate c-Jun, a component of the transcription factor AP-1. Blockade of c-Jun activation and transcriptional activity in vitro has been shown to prevent neuronal cell death (Estus et al., J. Cell Biol. 127:1717-1727, 1994; Ham et al., Neuron 14:927-939, 1995; Watson et al., J. Neurosci. 15:751-762, 1998). Recent data from c-Jun phosphorylation-deficient mice (Behrens et al., Nature Gen. 21:326-329, 1999) and from JNK knock-out mice (Yang et al., 1997) show that c-Jun phosphorylation is essential for injury-induced neuronal death.
Neurotrophic factors including NT-3, BDNF and GDNF have also been shown to be important for protection of neurons within the inner ear, and may also have a role in hair cell protection after cochlear insult (Gabaizadeh et al., Acta Otol. (Stockholm), 117:232-235, 1997; Ernfors et al. Ototoxicity: Basic Research and clinical applications, Savelletri di Fasano, Italy, Jun. 18-20, 1998, Abstract No. 12; Keithley et al., Neuroreport, 9: (10), 2183-2187, 1988; Shoji et al., ARO Meeting, St. Petersburg Beach, Fla., Abstract No. 539, 1998; Tay et al., ARO Meeting, St. Petersburg Beach, Fla., Abstract No. 538, 1998; Ylikoski et al., Hear Res 124:17-26, 1998). The loss of mechanoreception following cisplatin-induced neuropathy has been reversed through the administration of NT-3 (Gao et al., Arm. Neurol., 38:30-37, 1995).
TABLE 1TYPE OFTEST COMPOUNDLESIONREFERENCEantioxidants/free oxygen scavengersLipid peroxidationnoiseQuirk et al., 19941inhibitorR-phenylisopropanyl-noiseHu et al., 19972adenosineGlutathionegentamicinGaretz et al., 19943GlutathionenoiseYamasoba et al., 19984D-methioninecisplatinCampbell et al., 19965D-methionine (+BDNF)cisplatinGabaizadeh et al., 1997,supra.Na-thiosulfate (STS)cisplatinKaltenbach et al., 19976neurotrotrophic factorsBDNF (+D-methionine)cisplatinGabaizadeh et al.,1997, supra.NT-3 + MK801noise, amikacinErnfors et al., 1998, supra.GDNFcisplatinTay et al., 1998, supra.GDNFgentamicinShoji et al., 1998, supra.GDNFnoiseKeithley et al., 1998, supra.othersNMDA antagonistsaminoglycosidesBasile et al., 19967(MK801, ifenprodil)ORG 2766cisplatinDeGroot et al., 19978(ACTH analogue)Iron chelatorsgentamicinSong et al., 19979poly-l-aspartic acidgentamicinHulka et al., 199310Notated references from Table 1 (not cited previously).1Quirk et al., Hear. Res., 74: 217-220, 1994.2Hu et al., Hear. Res., 113: 198-206, 1997.3Garetz et al., Hear. Res., 77: 81-87, 1994.4Yamasoba et al., Hear. Res., 784: 82-90, 1998.5Campbell et al., Hear. Res., 102: 90-98, 1996.6Kaltenbach et al., Otol. Head Neck Surg., 117: 493-500, 1997.7Basile et al., Nal. Med., 2: 1338-1343, 1996.8DeGroot et al., Hear. Res., 106: 9-19. 1997.9Song et al., J. Pharmacol. Exp. Therapeutics, 282: 369-377, 1997.10Hulka et al., Am. J. Otol., 14: 352-356, 1993.
One problem in drug-based therapy of cochlear lesions is the limited biological activity of exogenously administered polypeptides. The biological half-life of many neurotrophic factors has been shown to be very short. On the other hand, degeneration of sensory hair cells does not occur instantly; a large number of sensory hair cells at first seem to be reversibly damaged and might recover if treated promptly. After noise exposure, the typical pattern of cellular damage in the organ of Corti takes 2-3 weeks to be complete. Affected cochlear neurons start to degenerate after noise has destroyed the sensory hair cells and the nerve terminals, 34 weeks postexposure.
There is no effective medical treatment to date for auditory sensory hair cell loss. Also, prevention of sensory hair cell degeneration is obscured by the fact that exact molecular mechanisms of damage to the auditory organ are unknown. Consequently, no effective regimen has been developed to prevent or treat damage to sensory hair cells. Therefore, there exists a need for compositions and methods to prevent and/or treat sensory hair cell damage.
There is also no effective medical treatment known to date for loss of cochlear neurons. Therefore, a need exists for compositions and methods to prevent and/or treat damage or loss of cochlear neurons.
As is clear from the foregoing discussion, damage to sensory hair cells or cochlear neurons can also affect the vestibular system and can result, for example, in vertigo. Benign paroxysmal positional vertigo (BPPV) affects about 40 to 60 people per 100,000 population every year. Also, Meniere's disease affects about 40 people per 100,000 population each year. During the course of these and other diseases, the sensory hair cells of the vestibular system have a tendency to degenerate. No effective regimen to date has been developed to prevent or treat damage to sensory hair cells in the vestibular system. Therefore, there exists a need for compositions and methods to prevent and/or treat damage to sensory hair cells and neurons in the vestibular system.